The present invention relates to a method and apparatus for reading invisible symbols (linear barcodes and two-dimensional symbols) usable in mail service, distribution of articles whose appearances are important, management of documents and articles requiring secrecy, and prevention of forgery.
Barcodes or two-dimensional symbols representing information about a manufacturing country, manufacturer, and product items are printed or pasted as labels on many articles currently distributed on the market. When an operator uses an optical reader to read a barcode, a corresponding price is read out from a database. This greatly reduces the working time compared to conventional work in which an operator inputs a price into a register. Stock control is also made efficient by constructing a POS system. Additionally, barcodes are effective to increase the efficiency of express delivery.
On the other hand, barcodes cannot be attached to some articles, and it is better not to attach barcodes to some articles presently having barcodes. For example, a printed barcode spoils the appearance of a book. This problem of appearance can be solved if an invisible or stealth barcode can be attached to an article. A tag or barcode is presently attached to the inside of a linen supply or clothing item where this tag or barcode is difficult to see. However, the work of distribution can be rationalized if an invisible barcode can be attached to the front surface of a packaged article.
In mail service, zip codes are read by an OCR to process a large amount of mails within a short time. However, this zip code reading is time-consuming and requires manual sorting of mails because read errors sometimes occur. Although barcodes may be used in mail service, visible barcodes cannot be printed on the surfaces of mails because the barcodes contaminate the mails. If information such as a zip code can be printed as an invisible barcode as in the above case, the time of sorting can be greatly reduced, and this allows rapid delivery of mails.
A barcode can contain much information in a narrow space. However, a barcode itself cannot unlimitedly shrink, so a fixed exclusive area is necessary. This exclusive area is not negligible if the size of an article is small. However, an invisible barcode can be superposed on some other printed information and hence does not require any exclusive area.
As a method of preventing forgery, invisible barcodes can be combined with another forgery preventing method. This may improve the effect of preventing forgery. As described above, invisible barcodes can extend the range of application of barcodes.
Two kinds of invisible barcode methods are presently possible: in one method a barcode is read by ultraviolet light, and in the other method a barcode is read by infrared light. In the method using ultraviolet light, a barcode is formed by using a fluorescent dye which does not absorb visible light. This fluorescent dye is excited by ultraviolet light, and fluorescence whose wavelength is different from that of the excitation light is detected. In the method using infrared light, a barcode is formed by using a metal complex which does not absorb visible light. This metal complex is excited by infrared light, and fluorescence whose wavelength is different from that of the excitation light is detected (xe2x80x9cStealth Barcodesxe2x80x9d, Tsunemi Ooiwa, OplusE, No. 213, p. 83, 1997).
Unfortunately, the ultraviolet light method has the following problem. That is, fluorescent dyes are often added to paper and cloth for bleaching purposes, and these fluorescent dyes also emit fluorescence. Since interaction with ultraviolet light is transition between electronic states of molecules, fluorescence less depends upon the intrinsic nature of a substance. Therefore, it is highly likely that reading of fluorescence emitted from an invisible barcode is interfered with. Also, a fluorescent dye in the ultraviolet region readily causes photo-deterioration, so it is highly possible that no predetermined fluorescence intensity can be obtained after a long-time use or storage. For these reasons, the reading accuracy largely declines easily.
Additionally, both fluorescent dyes and metal complexes have problems in toxicity and waste disposal. That is, barcodes are brought into homes together with commodities, and some barcodes remain existing in the living environment for long time periods. Therefore, babies and little children may lick these barcodes by mistake, or toxic low-concentration exposure to barcodes may occur. When these possibilities are taken into consideration, it is necessary to select materials from compounds already found to be safe. Furthermore, when recent waste disposal regulations are taken into consideration, it is desirable to select materials by taking account of even recycling and final disposal. In these respects, it is preferable to avoid the use of fluorescent dyes and metal complexes.
To prevent forgery, it is important that both a reader and an invisible barcode material be difficult to obtain. When an ultraviolet fluorescent dye is used, a light source for emitting ultraviolet light is readily available. Fluorescence in the visible light region can, of course, be visually checked. Fluorescence in the ultraviolet region is also easy to visually check by inputting the fluorescence into another material. Additionally, fluorescence less depends upon the intrinsic nature of a substance, so a substance which emits fluorescence similar to that of a visible barcode material is readily obtainable. On the other hand, in the infrared light method using a metal complex, an LED for the near infrared region can be used as a light source, and fluorescence can be detected by a CCD camera. Additionally, a similar fluorescent material can be easily obtained as in the case of a fluorescent material in the ultraviolet region.
As described above, an invisible barcode presently has many problems although it is expected as a technology meeting various needs.
It is an object of the present invention to provide a method and apparatus capable of reading invisible symbols with high reading accuracy.
A method for reading an invisible symbol of the present invention comprises the steps of heating an invisible symbol formed on a sample and containing a material which emits infrared light when heated, detecting infrared light emitted from the invisible symbol, calculating a differential coefficient of a detection signal corresponding to a position on the sample, determining, on the basis of upper and lower threshold values set for the differential coefficient, a maximum value of the differential coefficient in a region exceeding the upper threshold value and a minimum value of the differential coefficient in a region smaller than the lower threshold value, and binarizing the detection signal by using the maximum or minimum value as a leading or trailing edge of a binary function.
In the method of the present invention, it is also possible to heat the sample and detect infrared light emitted from the invisible symbol in a process of cooling the sample.
An apparatus for reading an invisible symbol of the present invention comprises heating means for heating an invisible symbol formed on a sample and containing a material which emits infrared light when heated, detecting means for detecting infrared light emitted from the invisible symbol, and an arithmetic operation unit for binarizing a detection signal from the detecting means. The arithmetic operation unit calculates a differential coefficient of a detection signal corresponding to a position on the sample, determines, on the basis of upper and lower threshold values set for the differential coefficient, a maximum value of the differential coefficient in a region exceeding the upper threshold value and a minimum value of the differential coefficient in a region smaller than the lower threshold value, and binarizes the maximum or minimum value as a leading or trailing edge of a binary function.
The apparatus of the present invention can further comprise means for moving the sample from a heating position of the heating means to a detection position of the detecting means, and control means for turning off the heating means heating the sample before detection by the detecting means. When these means are provided, infrared light emitted from the invisible symbol can be detected in a process of cooling the sample.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.